This invention relates to a full-duplex analog front-end circuit for interfacing between a digital signal-processing section of a modem and an analog signal line.
The purpose of a modem is to enable a digital device such as a computer to send and receive signals on an analog line such as a telephone line. In a full-duplex modem, separate frequency bands within an available or allotted band or channel, i.e,, a higher band and a lower band within the allotted channel, are assigned, one for transmitting and the other for receiving so that communication can take place simultaneously in both directions. Moreover, it is desirable that one can switch the modem between a first mode in which the higher frequency band is assigned for transmitting and the lower frequency band is assigned for receiving, and a second mode in which the lower frequency band is assigned for transmitting and the higher frequency band is assigned for receiving.
FIG. 1 is a block diagram showing an example of an analog front-end circuit for a full-duplex modem 8. The part enclosed in the dotted line is the analog front-end circuit 10. The analog front-end circuit 10 is connected on one side of the modem's digital signal-processing section 9 which in turn is connected to a digital device such as a computer 1, and on the other side to a pair of gain amplifiers 20 and 21, which are further connected to a hybrid transformer 23. The hybrid transformer 23 couples the gain amplifiers 20 and 21 to an analog signal line such as a telephone line 2 in such a way that signal transfer does not occur from the gain amplifier 20 to the gain amplifier 21.
The analog front-end circuit 10 comprises a digital-to-analog (D/A) converter 11 that receives digital signals from the digital signal-processing section 9 and converts them to analog signals, and provides the analog signal to transmission signal input terminal 31, a high-group bandpass filter 12 for restricting the output of either D/A converter 11 or amplifier 21 to the higher frequency band, a gain amplifier 13, provides the signal to transmission signal output terminal 32 a low-group bandpass filter 14 which eliminates frequency components outside the desired lower frequency band, and an analog-to-digital (A/D) converter 15 which receives signals via receive signal output terminal 33 converts analog signals to digital signals, which are furnished to the digital signal-processing section 9.
The output of the gain amplifier 13 is connected to the input of the gain amplifier 20. The output of the gain amplifier 20 is coupled through the hybrid transformer 23 to the telephone line 2. The analog signal received from the telephone line 2 via the hybrid transformer 23 is amplified by the gain amplifier 21 and supplied to receive signal input terminal 34.
The analog front-end circuit 10 further comprises switches 16 to 19 which are controlled by a signal F from a switch controller 7 to assume either the state illustrated or the state opposite to that illustrated, thereby to switch the modem 8 between the above-mentioned first mode and the above-mentioned second mode.
More specifically, when the switches 16 to 19 are in the state illustrated, the high-group bandpass filter 12 receives the output of the D/A converter 11, and the output of the high-group bandpass filter 12 is passed through the gain amplifiers 13 and 20 to the hybrid transformer 23, and low-group bandpass filter 14 receives analog signals from the hybrid transformer 23 through the gain amplifier 21 and the output of the low-group bandpass filter 14 is furnished to the A/D converter 15. In this way, the high-group bandpass filter 12 is used for transmitting and low-group bandpass filter 14 is used for receiving.
When the switches 16 to 19 are in the state opposite to that illustrated, the output of the D/A converter 11 is applied to the low-group bandpass filter 14, and the output of the low-group bandpass filter 14 is passed through the gain amplifiers 13 and 20 to the hybrid transformer 23. The high-group bandpass filter 12 receives analog signals from the hybrid transformer 23 through the gain amplifier 21, and the output of the high-group bandpass filter 12 is furnished to the A/D converter 15. In this way, the low-group bandpass filter 14 is used for transmitting and the high-group bandpass filter 12 is used for receiving.
The analog front-end circuit 10 further comprises elements not shown in the drawing, such as an automatic gain control (AGC) circuit for controlling the amplitude of the output of the filter 12 or 14 being used for receiving, a carrier detector for receiving the output of the filter 12 or 14 being used for receiving and generating a carrier signal when the output level exceeds a certain threshold, and an output driver for driving the output of the A/D converter 15.
Both the high-group bandpass filter 12 and the low-group bandpass filter 14 comprise a low-pass filter (LPF), a high-pass filter (HPF), and an all-pass filter (APF), not shown in FIG. 1, connected in series. The cut-off frequencies of the LPF and HPF determine the passband of the filter. The function of the APF is to equalize the group delay.
When an analog front-end circuit 10 is implemented in an integrated circuit, switched-capacitor filters are used for the LPF, HPF, and APF of the high-group bandpass filter 12 and the low-group bandpass filter 14, because switched-capacitor filters lend themselves to integration. A problem, however, is that a switched-capacitor filter generates unwanted second-order harmonics. Another problem is that a switched-capacitor filter generates a nonnegligible amount of output noise even when it does not receive an input signal. These problems tend to be especially severe for all-pass filters. This degrades the signal-to-noise (S/N) ratio of the signal.